This invention relates to a process for electroless plating of nickel or copper and is more particularly concerned with selective electroless plating of nickel or copper on a substrate. 2. Description of the Prior Art
In the electroless plating of nickel or copper on conductive or non-conductive surfaces it is often necessary to deposit the metal selectively only on certain predetermined areas. One of the reasons for applying nickel or copper in this manner is to take advantage of one or more of the unique properties of electroless nickel or copper such as their solderability, lubricity, chemical resistance and uniform buildup of plate. Another reason is a question of cost i.e., to avoid wasting nickel or copper on areas where they are not needed.
Various methods have been employed for masking of areas onto which deposition of nickel or copper is to be prohibited. One such method which has been employed widely is the application of so-called plater's tape such as that which is available from 3M Corporation. This method is mostly employed for plating of cylindrical or approximately cylindrical parts where the tape can be wrapped around and doubled over itself a number of times.
Another masking method commonly used employs a polymeric material such as paint, lacquer or the like which is applied to the areas to be masked by any appropriate means such as brush, stencil, screen printing and the like. Rubber and other elastomeric materials have also been employed particularly where the areas to be masked are the outside diameters of disc-like configurations or the inside diameters of annular shapes. Easy application and removal are the advantages of these types of material.
However, none of the above masking techniques is completely suited for use in the electroless application of nickel. Thus, engineering type electroless nickel plating processes are operated at very high temperatures, typically from about 180.degree. to 200.degree. F., and for prolonged periods up to about 24 hours or longer. Under such conditions plater's type tape, paint, lacquer and the like will soften and lose adhesion resulting in deposition of nickel on areas where it is not wanted. Removable masks such as rubber and other elastomeric materials have the disadvantage of allowing seepage of plating solution under the mask edges thus giving rise to irregular edge configurations. This seepage problem is aggravated by high temperatures, long deposition times and surfactant-containing plating solutions.
None of the above methods lends itself to the selective plating of electroless nickel or copper on small or complex patterns where the mask itself has to be applied in a finely detailed pattern and seepage under, or partial failure of, the mask cannot be tolerated.
It has now been found that if chromium is employed as a mask for the electroless deposition of nickel or copper, the problems noted above are circumvented. The use of chromium and other metals such as aluminum and nickel as a mask or resist in plating substrates with various metals has been described previously. Illustratively, Reilly U.S. Pat. No. 2,028,013 describes ornamenting an article by electroplating with chromium, cutting through the chromium layer to form an ornamental pattern and electroplating gold on the exposed substrate. Barnhart U.S. Pat. No. 1,996,187 shows a process of masking metal plated articles in which a layer of chromium is deposited on a base such as nickel, a pattern is etched in the chromium, and a metal of dissimilar color to the chromium (e.g. gold, copper, silver, nickel) is electroplated on to the exposed metal of the base.
Brown et al. U.S. Pat. No. 3,809,625 describe a process for using chromium as a mask in making compact bumps on a semi-conductor wafer. The process comprises forming a metallization pattern on the wafer, providing a continuous film of glass over the pattern with holes in the film over contact pads, depositing a layer of chromium over the glass, forming contact pads of metal e.g. gold on the chromium layer over the holes in the glass, depositing bump metal e.g. silver on to the pads, then removing the exposed portions of the chromium layer.
Russell U.S. Pat. No. 2,367,314 shows the use of nickel as a masking material for selectively electroplating gold on a conductive substrate.
Russell U.S. Pat. No. 3,948,736 teaches the selective electroplating (as opposed to electroless plating) of precious metals on a substrate using metals such as aluminum as the masking material.
The use of electroplated chromium as a mask for the electroless deposition of nickel or copper on selected areas of a conductive substrate has not been described or suggested heretofore and the advantages which flow therefrom have gone unrecognized.